fix: propagate cumulative gas overflow in block executor (#42 follow-up)

[crazywriter1]

Summary

Completes the defensive gas accounting started in #42 and extended in #135. ArcBlockExecutor::commit_transaction still used checked_add(...).expect("cumulative gas overflow"), which would abort block execution if cumulative gas accounting ever overflowed.

Although this overflow is theoretically unreachable in production (gas_used is bounded by the block gas limit, far below u64::MAX), propagating the error is consistent with the project's existing overflow-handling strategy and removes an unnecessary panic path.

Changes

crates/evm/src/executor.rs

• Add ERR_CUMULATIVE_GAS_OVERFLOW.
• Replace checked_add(...).expect("cumulative gas overflow") with ok_or_else(|| BlockExecutionError::msg(ERR_CUMULATIVE_GAS_OVERFLOW))?.
• Return a BlockExecutionError instead of panicking if cumulative gas accounting overflows.

Context

• fix: use checked arithmetic for cumulative gas accounting in payload builder #24 — reviewer requested explicit error propagation instead of panic/silent handling for cumulative gas accounting.
• fix: use checked arithmetic for cumulative gas accounting in payload builder #42 — applied the same pattern in the payload builder.
• fix: complete payload builder gas overflow error propagation #135 — replaced .expect() with proper error propagation in the payload builder (complementary work).

This PR applies the same defensive overflow handling to the block execution path, keeping overflow behavior consistent across the codebase.

Test Plan

• cargo fmt -p arc-evm -- --check
• cargo clippy -p arc-evm --all-targets -- -D warnings
• cargo test -p arc-evm